The present application relates to an apparatus and method for performing membrane gas/liquid absorption at elevated pressure.
Methods for membrane gas/liquid absorption are known, for example from Applicant""s patent applications mentioned below. These processes generally comprise the absorption of one or more gaseous components from a gas phase, the gas phase containing the components to be absorbed being brought into contact with a liquid phase, the gas phase and the liquid phase being kept separate by a membrane.
Membrane gas/liquid absorption is a very flexible and versatile technique, which can be used for specific absorption of diverse compounds from a gas phase, depending on, inter alia, the membrane used, the liquid phase used, and the gas stream to be cleaned. For example, membrane gas/liquid absorption can be used for specifically absorbing carbon dioxide and H2S (European application 0,751,815), oxidizable and reducible constituents such as mercury vapour (PCT application NL 96/00279), and also for removing water vapour (European application 0,524,242 by Applicant).
The known techniques have the drawback, however, that they cannot be applied to/with gas streams at elevated pressure, in particular pressures of more than 4 bar. Nevertheless it would be highly advantageous to be able to carry out membrane gas/liquid absorption at elevated pressure from the gas phase, particularly in those cases where the gas phase is provided at elevated pressure and where it is not possible/desirable, for technical and/or economical reasons, to reduce the gas stream pressure prior to the absorption process. An example of this is refining of natural gas which, as a rule, is produced, and has to be processed, at a pressure of more than 50 bar and sometimes 100 to 200 bar.
Existing membrane gas/liquid absorption techniques cannot, however, be applied to gas streams having such a high pressure, notably because:
existing equipment for gas/liquid membrane absorption is not designed for such high pressures;
existing membranes, particularly porous membranes, cannot be used if there is a pressure drop across the membrane;
if xe2x80x9cclosedxe2x80x9d membranes that can withstand a high(er) pressure drop (i.e. having a suitable thickness) are used, the mass transfer and consequently the capacity may become too low.
The invention therefore relates to an apparatus for performing membrane gas/liquid absorption at elevated pressure, comprising:
a pressure vessel (1) which encloses an essentially closed chamber (2);
means for supplying (3a) and discharging (4a) a gas phase to/from chamber (2);
a membrane unit (5) disposed in chamber (2), comprising at least one membrane element (6) which defines a feed-through channel (7);
means for supplying (8a) or discharging (9a) a liquid phase to the membrane unit (5), such that the liquid phase can be passed from inlet (8a) through feed-through channel (7) to outlet (9a);
wherein pressure vessel (1), inlet (3a) and outlet (4a) and membrane unit (5) are provided in such a way in chamber (2) that the gas phase can be directed past membrane element (6) in a flow direction essentially perpendicular to the flow direction of the liquid phase through membrane element (6), exchange of components to be absorbed being able to take place between the gas phase and the liquid phase through (the wall of) membrane element (6).
The invention further relates to a method for performing membrane gas/liquid absorption at elevated pressure for absorbing one or more components from a gas phase, employing the above-described apparatus, comprising
feed-through of the gas phase comprising the one or more components to be absorbed, via inlet (3a) and outlet (4a) through chamber (2) past the one or more membrane elements (6), the gas phase having a pressure of more than 4 bar, preferably more than 10 bar, more preferably 50-200 bar;
feed-through of a liquid phase suitable for absorbing the one or more components, via inlet (8a) and outlet (9a) through feed-through channel (7), the liquid phase having a pressure which differs from the gas phase pressure by not more than 5 bar, preferably not more than 0.5 bar;
in such a way that the one or more components to be absorbed are absorbed from the gas phase into the liquid phase through (the wall of) the one or more membrane elements (6), the gas phase and the liquid phase being kept separate by the membrane elements (6).
According to the invention, the membrane gas/liquid absorption is performed with so-called xe2x80x9ccross-flowxe2x80x9d, i.e. the direction of the gas phase containing the one or more components to be absorbed is perpendicular to the plane of flow of the liquid phase.
Pressure vessel (1) is preferably essentially cylindrical or encloses an essentially cylindrical chamber (2). Whilst, in the operating position shown, the longitudinal axis of pressure vessel (1) is arranged essentially vertically, other setups are also possible, since membrane absorption is essentially independent of the orientation of the pressure vessel. These additional degrees of freedom regarding the set-up of the membrane absorber are an important advantage of membrane gas absorption compared with (for example) the use of packed columns which operate under the influence of gravity and therefore always have to be operated essentially vertically to achieve the desired counterflow.
Pressure vessel (1) may comprise guide means for controlling the flow of the gas phase through chamber (2).
Membrane element (6) has such a shape that it defines at least a feed-through channel (7). As such, membrane element (6) may consist of one membrane or of an assembly of a plurality of membranes, which form/enclose the feed-through channel; for example, membrane unit (5) may consist of planar membranes having transport channels, membranes that form a so-called xe2x80x9cplate-and-framexe2x80x9d module or are in the form of spirally coiled membranes. Membrane element (6) is preferably in the form of a hollow fibre.
The membranes can be made of any suitable material which is at least permeable for the one or more gaseous constituents to be absorbed, but not for the gas phase and the liquid phase.
The membranes are preferably inert and able to withstand the gas phase and liquid phase used and the constituents to be absorbed, and are further selected on the basis of the intended use and further factors such as the desired mass transfer. In this context it is also possible to employ selective membranes.
The membranes can be either porous and nonporous, and can be asymmetric and/or coated membranes. It will be evident to those skilled in the art that, to achieve high mass transfer, porous membranes are to be preferred as a rule; these, however, are most sensitive to a pressure drop across the membrane.
Suitable membrane materials are known from the prior art, such as the abovementioned applications by Applicant, and comprise porous membranes such as polypropylene (PP), polyethylene (PE), poly(vinylidene fluoride) (PVDF), poly(tetrafluoroethylene) (PTFE) and polysulphone (PSU); nonporous membrane materials, asymmetric and/or coated membranes such as plasma membranes, membranes coated with siloxane rubbers (PDMS), membranes treated with fluorine, paraffin waxes and the like. Other suitable materials are, for example, ceramic membranes (Al2O3, TiO2, ZrO2), cellulose acetate (CA), butadiene rubber, EPDM (ethylene/propylene terpolymer); metallic membranes (e.g. Pd), poly(phenylene oxide) (PPO), polyimide (PI) and the like. Other materials will be obvious to those skilled in the art.
The membrane unit (5) comprises at least the one or more membrane elements (6) and possibly further means, such as guide elements for guiding the gas stream past the membrane elements (6), for example wall elements which enclose the membrane unit parallel to the gas flow direction; and/or connecting and distributing elements for distributing the (stream of the) liquid phase over the throughflow channels (7) of membrane elements which may, for example, be linked to, in particular form part of, the guide elements, as described below in more detail.
The membrane unit (5) preferably comprises an assembly of a plurality of hollow fibres which essentially run parallel, where the number of the hollow fibres and their total exchange area will depend on the capacity to be achieved.
In this context it is possible for various assemblies (modules) of hollow fibres to be used, which may or may not be operably connected to one another, with the option of the various assemblies of hollow fibres being at an angle with respect to one another, preferably an essentially perpendicular angle, all in planes perpendicular to the flow direction of the gas phase.
The flow direction of the gas phase past the membrane elements (6) is preferably essentially parallel to the longitudinal axis of pressure vessel (1), which means that the membrane elements (6) will be situated in a plane essentially perpendicular to the longitudinal axis of the pressure vessel (1).
The design of the absorber is preferably such that it is possible for the absorption process to be carried out as a countercurrent process, as will be known to those skilled in the art.
The membrane unit (5) is preferably a DAM module as described in the international application 91/09668 by Applicant, which is incorporated herein by reference.
Implementing the method of the invention involves passing the gas phase which contains the one or more components to be absorbed through chamber (2) past the membrane elements (6), a liquid phase suitable for absorbing the one or more components being passed, essentially simultaneously, through feed-through channel (7) of membrane element (6), in such a way that the one or more components to be absorbed are absorbed from the gas phase into the liquid phase, the gas phase and the liquid phase being kept separate by the (wall of) membrane elements (6).
During this process the gas phase in chamber (2) has a pressure of more than 4 bar, preferably more than 10 bar, more preferably 50-200 bar.
The pressure of the liquid phase in feed-through channels (7) will be essentially equal to the pressure of the gas phase in chamber (2), i.e. not differ therefrom by more than 5 bar, preferably not by more than 0.5 bar. Highly preferably, the liquid phase is at slight excess pressure relative to the gas phase, i.e. within the abovementioned range.
The pressure of the liquid phase will generally be set as a function of the pressure of the supplied gas phase, by means of suitable pressure controlling means. These pressure controlling means are preferably linked to, and are more preferably provided in, the absorber itself, more preferably to or in, respectively, pressure vessel (1), as is described below in more detail in the preferred embodiment.
The pressure controlling means are preferably such that they are able to respond suitably to changes in the pressure of the liquid phase, i.e. that they are able to compensate for such a pressure change by a corresponding change in the pressure of the liquid phase, without the integrity of the process and/or the equipment used being affected.
An advantage of the invention is that the membrane elements (6) and the membrane unit (5) as such need not be suitable for use at high pressure or pressure drop. Pressure vessel (1) and the further connections and the like do, however, have to be able to withstand the pressure of the gas phase and liquid phase, as will be obvious to those skilled in the art.
A further advantage of the (design of the) absorber according to the invention is, for example, that the pressure vessel can readily be filled and the membrane unit can readily be replaced, inter alia as a result of the modular structure. This leads to fewer operations in the course of maintenance and replacement.
The high-pressure membrane absorber according to the invention can be used for performing any gas/liquid absorption processes known per se at elevated pressure of the gas phase, in particular known membrane gas/liquid absorption processes, for example the CO2/H2S absorption of the European application 0,751,815 by Applicant, the oxidative membrane gas/liquid absorption of the international application NL 96/00279 by Applicant, and the water vapour removal of the European application 0,524,242 by Applicant, whose contents are incorporated herein by reference.
In this context, said processes will, as a rule, be carried out in a manner similar to that of the known processes, i.e. using essentially the same membrane materials, liquid phases, absorption media, regeneration techniques for the liquid phase and the like. The invention therefore makes it possible for these known processes to be adapted, in a simple manner, for use at elevated pressure of the gas phase, by using the apparatus and method described herein, and the uses of these known processes at elevated pressure form preferred aspects of the invention.
According to a specific embodiment, the absorber may also comprise at least two separate membrane units (5), which can be used for a plurality of identical or different liquid phases being fed simultaneously through the membrane elements (6) linked to these membrane units. Said separate membrane units in this arrangement will each effectively be connected to corresponding separate means for supplying (8a) or discharging (9a) the respective liquid phases. This embodiment makes it possible, inter alia, by employing a plurality of suitable liquid phases different from one another, to subject a gas phase in a single absorber simultaneously to different absorption processes for the simultaneous removal of different gaseous components, moreover providing the option of these absorption processes being controlled separately.
In this embodiment, said separate membrane units (5) may form part of a single design in the absorber, as long as it is possible for the different liquid phases to be passed, independently of one another, through the separate membrane units.
It is also possible for a plurality of membrane gas/liquid absorbers according to the invention to be connected in series, for example using the same absorption fluid to increase the efficiency of the removal, or using different absorption fluids to remove different components in series.
The liquid phase containing the components absorbed from the gas phase can be regenerated in accordance with processes known per se and can be reused for absorption. Thus the invention can be used in a closed liquid-phase circuit, optionally provided with suitable pumping means, the liquid phase during a cycle passing through both an absorption step and a desorption step. The desorption/regeneration step in this process can be carried out at the same liquid-phase pressure as the absorption step, using suitable equipment; in the process it is possible, if appropriate, in/for carrying out the desorption process, to advantageously utilize the elevated pressure of the liquid phase and/or the components incorporated therein, especially in the case of desorption of gaseous components, as will be obvious to those skilled in the art. Equally it is possible for the desorption to be carried out at a lower liquid-phase pressure (compared with the absorption step), with the option of suitable means for raising and lowering the liquid-phase pressure being incorporated in the liquid circuit.
According to a preferred aspect of the invention, which will be discussed below in more detail, it is possible for the (particularly gaseous) components absorbed from the gas phase to be desorbed/extracted during the desorption/regeneration step from the liquid phase as a gas (stream) having a higher (partial) pressure than the (partial) pressure at which the components were originally present in the gas phase, or even having a higher absolute pressure than the gas phase. This can take place by means of raising the desorption temperature. This so-called xe2x80x9cpump effectxe2x80x9d provides advantages, in particular, in refining natural gas according to the invention, as described below in more detail.
A further preferred aspect of the invention, which likewise will be described below in more detail, relates to a specific method for desorbing sulphur compounds absorbed into the liquid phase, in particular gaseous sulphur compounds such as, in particular, H2S, by means of a redox reaction, in the course of which the sulphur can, for example, be precipitated as a free compound.
Possible applications of the membrane gas/liquid absorber and method according to the invention are:
absorption of CO2 from gas streams at high pressure such as fuel gas, natural gas and associated gas;
absorption of H2S from gas streams at high pressure such as fuel gas, natural gas and associated gas;
absorption of Hg from natural gas, petroleum (fractions) or natural gas condensates (hydrocarbons that are gaseous or provided in gaseous form);
absorption of mercaptans or acidic components from petroleum (fractions) or natural gas condensates (hydrocarbons that are gaseous or provided in gaseous form);
absorption of water (vapour) from natural gas, petroleum (fractions), LPG (hydrocarbons that are gaseous or provided in gaseous form);
extraction/refining of alkenes (ethylene, propylene, butylene, styrene), for example in petrochemical processes;
absorption and/or extraction of CO from synthesis gas or waste gases at high pressure.
According to a particular, highly suitable embodiment the invention is used for refining natural gas, in particular for removing constituents such as mercury, CO2, H2S from natural gas and/or drying natural gas.
It is known that natural gas is produced at pressures of more than 50 bar, as a rule approximately 100-250 bar. Freshly produced natural gas contains considerable amounts of CO2 (more than 10 bar), H2S, Hg and water vapour.
Reducing the pressure of the natural gas prior to the membrane gas absorption process is technically difficult and uneconomic, partly because natural gas, after refining and in a subsequent step, has its pressure boosted or is liquefied, for example for the purpose of transport. This involves high(er) energy costs and investment costs (compressors and cryogenic processes) at a lower inlet pressure of the natural gas. Moreover, liquefaction of natural gas or propane (for LPG) can be carried out only with great difficulty if the initial gas contains considerable quantities of CO2. A method for refining natural gas at high pressure, preferably the pressure at which the natural gas is produced from the field, is therefore highly desirable.
The invention therefore relates, in particular, to a method for refining a stream of natural gas at elevated pressure, wherein the stream of natural gas is selectively stripped of one or more gaseous impurities by absorption of the one or more impurities into a liquid phase, by employing an apparatus as described above or in accordance with a method as described above.
This aspect of the invention particularly relates to certain methods for refining natural gas where:
the impurity to be removed is CO2 and/or H2S, the liquid phase used being an aqueous solution of an absorption medium for CO2 and/or H2S. In this context, the membrane elements (6) preferably comprise hollow fibre membranes made of polypropylene, polyethylene, polysulphone, poly(vinylidene fluoride) or poly(tetrafluoroethylene) and the liquid phase is as described in the European application 0,751,815 by Applicant, i.e. an aqueous liquid having a surface tension at 20xc2x0 C. of more than 60xc3x9710xe2x88x923 N/m, more preferably an aqueous solution of a water-soluble amino acid or salt of an amino acid, more particularly taurine, alanine, glycine, methylglycine, proline, dimethylglycine, a salt or a derivative thereof; or phosphate/carbonate salts or phenolate;
the impurity to be removed is mercury (vapour), the liquid phase used being an aqueous solution of an oxidant suitable for oxidizing mercury, as described in the international application NL 96/00279 by Applicant; and/or
water (vapour) is removed, the liquid phase used being a hygroscopic salt solution as described in the European application 0,524,242,
or a combination thereof, preferably implemented in a plurality of membrane absorbers connected in series, as described above.
Those skilled in the art will be able to ensure that the abovementioned absorption fluids are optimally adapted to the absorption of the constituents from natural gas.
With respect to refining natural gas, the invention additionally comprises a special aspect, viz. the extraction, during the desorption/regeneration process, at a higher (partial or absolute) pressure than the absolute pressure of the stream of natural gas and/or the partial pressure of the gaseous constituents in the stream of natural gas, of the gaseous constituents such as CO2 and/or H2S, in particular CO2, absorbed from the stream of natural gas into the liquid phase. Said higher pressure makes it possible for the gas stream which is obtained from the absorption/regeneration process and which contains a predominant amount of CO2 to be (re-)injected into the ground, for example into the gas field itself, this being considerably preferable, from an environmental point of view, to discharge into the atmosphere. The high-pressure gas stream obtained from the process can also be used in other ways known per se, especially in situ in the gas field or the refinery, for example as a displacement gas used underground, as an adiabatic refrigerant or for driving turbines.
In refining natural gas it is also possible, advantageously, to employ oxidative sulphur absorption as described herein.